1. Field of Invention
The invention relates to vehicle steering systems, with particular regard to improvement of high-speed stability by dynamic transient torques from an electric or hydraulic actuator. This function of a steering apparatus is different from, and can act in addition to, the usual power steering function of assisting the driver's steering action. The invention helps the driver to maintain control of the vehicle during severe emergency maneuvers, with front-wheel-steering (FWS) but also with rear-wheel-steering (RWS) which is otherwise unstable at high speeds.
2. Discussion of Prior Art
Power steering systems generally function by amplifying the torque exerted on the steering wheel by the driver, so that the steering wheel torque required of the driver is less than the actual torque needed to steer the wheels, with the extra torque being provided by a hydraulic or electric actuator. A recent example of a power steering system which functions as a torque amplifier is U.S. Pat. No. 4,621,701. It has been recognized that the amplification gain of power steering should decrease as vehicle forward speed increases because a driver can more easily lose control at high speeds if rapid steering motions are effortless. In accordance with this level of understanding, speed-dependent power steering has been implemented. For example, U.S. Pat. No. 4,640,380 describes an apparatus which applies opposing torque to the steering wheel in a fashion that increases with vehicle speed.
The prior art for the design of steering systems has not recognized that automobiles have a natural mode of steering oscillation at a frequency of approximately 1 Hz, a phenomenon which becomes more pronounced at high speeds. This weave mode consists of a lateral oscillatory motion of the entire vehicle as it moves along the highway, but it is usually not noticed because of steering system friction and the fact that driver steering frequencies are almost always below 0.5 Hz. Although the weave mode has been documented in the scientific literature (Segel 1965), its cause and importance have not been understood.
Drivers must steer rapidly for emergency avoidance lane change maneuvers which require steering frequencies up to and above 1 Hz, and steering torques much greater than steering system friction. The weave mode resonance at about 1 Hz therefore becomes excited, which compromises the driver's control authority over the vehicle. It is well known that loss of control in such situations is characterized by overshoot of steering corrections and lateral oscillation of the vehicle position on the roadway. The cause of the weave mode can be understood by considering the source of natural steering torque in the response of vehicles.
When a vehicle with a conventional steering system is in a steady turn, there is a restoring steering torque due to the lateral tire force applied by the road toward the center of the turning circle. The caster offset or trail of the steering geometry provides a lever arm for action of this force about the kingpin axis, so a torque is produced which tends to return the steered wheels to the straight-ahead position. However, there is a delay in the change of restoring steering torque after a change in steer angle. This delay occurs either in the presence or absence of a power steering apparatus because it is directly related to a vehicle's lateral acceleration response time.
After performing a rapid emergency lane change, a driver must quickly reverse the steering direction in order to direct the path of the vehicle straight along the new lane. At high speeds, the natural restoring steering torque persists after the steer angle reversal occurs, causing overshoot in the new steering direction. After the characteristic delay time, the natural restoring torque then reverses direction also, but not until after significant overshoot has occurred. The resulting oscillation is the weave mode, and the driver is in danger of losing control of the vehicle.
Prior art vehicle steering systems have sources of steering torque in addition to tire forces, including power-assist and steering dampers. Conventional power steering systems cannot prevent excitation of the weave mode by the driver, because the magnitude of power-assist torque is not sensitive to the dynamic state of the vehicle which causes the delay in natural steering torque. Essentially, the torque applied by a power steering actuator does not have the transient character which is needed to compensate for the weave mode resonance. Conventional steering dampers eliminate steering system vibrations because they apply opposing effort depending on steering angular rate. However, the weave mode is a whole vehicle mode fundamentally different from any steering system vibration. Dampers are not sensitive to the dynamic state of the vehicle so only excessive levels of damping begin to affect the weave mode.
In the case of rear-wheel-steering (RWS) vehicles, there is not merely a delay in buildup of steady-state restoring torque, there is a transient torque component which drives the steering mechanism in the direction the steering wheel is turned, resulting in an unstable weave mode rather than a lightly damped stable weave mode. The prior art includes details for the construction of RWS motor vehicles without means for high speed stabilization (U.S. Pat. No. 2,101,057). No practical high-speed RWS motor vehicles have heretofore existed because passive stabilization means cannot compensate for changes in vehicle speed. For example, the prior art by Whitehead (1983) indicates that passive stabilization can be achieved by extreme levels of steering damping, which is impractical because the excessive damping required at high speed impedes driver steering motions.